In the field of forensic science, criminal investigators are seeking improved methods to detect evidence. Such evidence includes blood, saliva, semen, and other body fluids; as well as hair, flesh, bone fragments, teeth, human skin damage such as bruises, bite marks or cuts; shoe prints, fingerprints, footprints, tire prints, gunpowder residue, bullets, explosives, chemical and biological agents, paint, grease or oil, glass fragments, fibers and various trace evidence, including the alteration of documents.
The detection of evidence has historically been a combined process of art and science. One conventional method of obtaining, for example, fingerprint evidence is the careful lifting of fingerprints by applying a fine dust to the surface of a fresh print and then transferring the dust pattern onto a second surface. Fingerprint dusting powders were initially selected for their color contrasting qualities. Extremely fine fluorescent dusting powders were also used to visualize minute etchings of a surface caused by the breakdown of amino acids contained in fingerprint oils. The fluorescent dusting powder adheres to the etchings and reveals the fingerprint pattern upon illumination by ultraviolet radiation. Other substances, such as blood, saliva or semen, are easily detected where visible stains exist. However, often such revealing evidence is concealed from ordinary inspection via cleansing agents or even the passage of time.
In more recent years, ultraviolet light has been used by forensic specialists to aid in the viewing of otherwise invisible evidence. Ultraviolet (“UV”) radiation is light that is just beyond the visible spectrum. Whereas visible light has a wavelength ranging from about 400 nm to about 750 nm, UV radiation has a shorter wavelength and ranges from about 10 nm to about 400 nm. Although the unaided human eye cannot discern UV radiation, its presence can be shown by use of either UV-sensitive media or the resultant fluorescence of a UV-sensitive material. When fingerprints are dusted with fluorescent dusting powders, applied UV radiation will result in the fluorescing of the dusting powder, effectively making the fingerprints visible to the unaided eye. Other organic substances, such as saliva or semen, naturally fluoresce, and are thus also visible upon exposure to UV radiation.
Conventional UV fluorescence detection methods involve using a so called “black light” in the form of a continuous or non-flash flood-type UV source to continuously generate UV radiation near the evidence, and then using a camera with a filtered lens to capture the resultant fluorescent image onto film. Because the evidence will fluoresce at a relatively low intensity, light sensitive film and a relatively slow shutter speed are required. Additionally, the camera must also be held very close to the evidence. The effect, then, is a relatively narrow field scan that must be repeated many times to effectively detect evidence over a large area.
Variations on conventional UV fluorescence detection methods include the use of chemical reagents or vapors to deposit fluorescence-emitting particles onto fingerprint etchings where regular dusting methods are not appropriate. A laser, instead of a black light, may also be used to excite the fluorescence-emitting material. However, laser excitation is, by its very nature, limited to use over a small overall area. Additionally, the portability and use of a laser in small areas may be limited due to the large power supply required for operation of the laser.
A more recent UV detection method utilizes UV reflectance instead of fluorescence. Although the reflected UV detection method may not result in the clean images produced by UV fluorescence, high-intensity short-wave UV reflectance can reveal otherwise undetectable or difficult to detect evidence on non-porous surfaces that limit the use of fluorescent dusts or dyes.
Analysis of evidence detected through UV fluorescence or reflectance generally includes the development and close examination of photographs taken of the UV-detected evidence to determine the exact locations of the evidence. Dimensions of the detected evidence and overall shape are often obtained by hand measurement or even sketched reproduction. The substance of the evidence is determined by DNA testing as well as by other tests for specific reagents.